In order to plan and manage mining operations and to estimate recoverable ore reserves it is necessary to have the facility to determine the concentration in ore samples of precious metals, typically gold and PGMs (platinum group metals including platinum, palladium, rhodium, osmium, indium and ruthenium). It is already known to use a fire assay process for this purpose. In fire assay, PGMs and gold are separated from gangue materials by collection into either lead or nickel sulphide at temperatures of around 1200–1450° C. This is achieved by mixing an aliquot of the sample with a flux containing either lead oxide, for the lead collection, or a combination of nickel carbonate and sulphur, for the nickel sulphide, with other chemicals. A flux containing lead oxide which has been found to work satisfactorily in the conventional fire assay method consists of calcium carbonate, lead oxide, borax and silica, and a carbon source such as activated carbon, maize meal or flour. This mixture is placed into a ceramic crucible, which in turn is placed into an electric or gas fired furnace and heated to an appropriate temperature for a period of about 90 minutes. During this time the mixture melts and, because their chemical affinity, PGMs and gold are collected into either lead of nickel sulphide. On cooling the lead or nickel sulphide is separated from the gangue material and the PGMs and gold content determined by a number of analytical techniques.
The advantage of Fire Assay collection over other analytical procedures is that it effectively concentrates PGMs and gold from a large sample aliquot into a media which is much more amenable to further treatment for the separation and analysis of the PGMs and gold. Fire Assay is, therefore, used extensively in all laboratories engaged in the analysis of samples containing precious metals and, indeed, is an essential stage in the analysis of lower grade samples such as concentrates, feeds and tails.
In electrically heated, furnaces, radiant heat from resistive elements is applied to the prepared ore samples. It is difficult to maintain constant operating conditions in an electrical furnace. Furthermore, resistive elements are fragile and deteriorate over time. This variability in furnace conditions detrimentally affects the accuracy of assay results and gives rise to excessive repeat rates.
Gas fired furnaces on the other hand are noisy and require frequent maintenance, which lengthens the average time taken to complete an assay.
The flux used in assaying contains lead and is environmentally hazardous, either in dust form during the flux preparation stage, or as fumes which form during the fusion and cuppelation stages of the process. Personnel involved in the assaying process require monitoring of lead blood levels every six months, which is expensive and disconcerting to the persons involved.
The fusion pots used for fusion of the flux/ore sample combinations are brittle and break easily. In order to overcome this difficulty, each assay is duplicated or triplicated, to ensure that at least one successful assay result is obtained, which increases the overall assaying costs. Further, the duplicated or triplicated samples are fused in different furnaces in order to compensate for the variability in furnace conditions.
The recovery of lead from the fusion slag is also hazardous as the slag is broken tip, usually manually, by impact to liberate globules of lead entrapped in the slag. Slivers of slag are sharp and necessitate the wearing of adequate safety equipment. Generally, not all lead globules are usually liberated from the slag, leading to an inevitable loss of lead.
Current assay techniques are labour intensive and, therefore, prone to human error. The average time taken to complete an assay normally exceeds twelve hours. It is desired to improve the accuracy, the turnaround time and the safety aspects of known prior art assaying techniques.